What does a lathe's chuck do?

The chuck of a lathe serves the essential purpose of gripping a workpiece securely for machining. It consists of a set of jaws that can expand or contract to hold different sizes and shapes of materials in place, allowing for precise cuts and operations to be performed. This is crucial in machining because stability of the workpiece is vital in achieving a uniform and accurate finish. By securely holding the workpiece, the chuck minimizes vibration and movement, which ensures that the machining process yields the desired dimensions and surface finish.

Other functions such as controlling the speed of the lathe, measuring depth during cutting, or holding the cutting tool are performed by different components of the lathe and do not involve the chuck's function. Understanding the role of the chuck helps clarify its importance in the overall operation of a lathe, making it clear why this choice is the best answer.